GRA said:
As has been discussed previously at length, sustainable biofuels require EROEI above unity,...
Yes, I have posted on this topic extensively and, no, getting above unity EROEI is NOT sufficient to make biofuels sustainable. Getting above an EROEI of 1:1 ONLY allows us to reduce the amount of fossil fuels consumed for the ongoing production of biofuels. In order to make biofuels sustainable, you not only need to pay back the energy used to make the biofuels, but you also need to pay back all of the energy used to build all of the infrastructure needed to create the biofuels: tractors, biofuel processing plants, etc. That gets the minimum EROEI up to around 3:1. But at 3:1 EROEI, you find that you do not have sufficient energy left over to grow the industry at a non-zero rate. As a result, the energy needed to grow the industry comes from non-sustainable resources.. To do that, you need to get EROEI much higher, like above 10.
Are there biofuels out there with an EROEI above 10? Frankly, I don't think that is even on the horizon.
And then you have the massive land-use issues. No, algal-based biofuels do not address this issue since they produce such minute amounts of biofuel.
As I said, biofuels solve no problems. They only add to existing ones. Burning biofuels in cars is hurting our environment. Burning it in airplanes will hurt our environment even more.
That brings us back to PV. EROEI is currently above 10 (by how much is the only matter of debate). Land use required for PV for transportation is minuscule when compared with biofuels, as noted. And that goes down as efficiency improves. 20% sunlight-electricity conversion efficiency is likely far beyond anything biofuels could ever hope to achieve.
So, how do you fly airplanes on PV?
- Batteries? This is the most efficient approach, but batteries do not get lighter as they discharge, so they have a distinct disadvantage for long-haul flight. I expect battery-powered flight to address short range needs in the future, but I have trouble envisioning it for long-range flight or cargo. This is similar to my views of BEVs for land transportation.
- Flow batteries? Possibly, but these also do not get lighter as the flight progresses. I'm not sure what the energy cost is, either.
- Hydrogen? In liquid form, hydrogen contains 3X the energy of hydrocarbon fuels by weight. Unfortunately, it takes up 4X the volume. Also, liquid hydrogen is extremely unlikely to stay liquid in a crash, which likely will cause some pretty spectacular fireballs when liquid-hydrogen-fueled airplanes crash. On top of all that, production of hydrogen will immediately cut the EROEI of PV by over half. Perhaps to less than 1/3. So the question is whether or not there is a way to store sufficient hydrogen in a safe, lightweight container for flight. Metal hydrides are not light and are energy-intensive.
- Some other liquid fuel? This may end up being the ultimate solution. But most liquid fuels require significant energy to manufacture. This will improve as technology advances, but who knows how long it will take?